Keep supporting the env / make variable PERL

OpenSSL 1.1.0 supports the use of this environment variable for
passing to the build files.  For the sake of backward compatibility,
we keep it.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/6668)